Tim Williams wrote:

"Michael A. Terrell" wrote in message
...
dB without a reference is meaningless. How can you have a ratio
without a reference?

Easy. "dB" in general usually refers to acoustic power, where the reference
is some ungodly small power level (10^-12W/m^2 IIRC?).

I forget if there's a similar radio context used...


You need to do a lot of studying on how to use the dB. Without a
reference, it is meaningless. The classic use in audio was 1 mW into
600 Ohms = 0 dBm, and yes, there are a number of RF uses for the dB.
Either in reference to one of several different reference levels, or as
an absolute ratio, such as the input to output level of an amplifier.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

"Michael A. Terrell" wrote in message
...
You need to do a lot of studying on how to use the dB.

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

In article ,
"Tim Williams" wrote:

"Michael A. Terrell" wrote in message
...
You need to do a lot of studying on how to use the dB.

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.

Like Michael stated dB is a logarithmic reference-less ratio value.

You can use dB for things like amplifiers that have a gain or
attenuators that have a loss for example. The gain of an amplifier can
be expressed in dB because the reference value is the input value of
the amplifier, which will allow you to calculate the output value but
if you are speaking of a value of power or voltage by itself then you
need an absolute scale with a reference quantity.

Absolute scales would be dBV, dBuV, and dBm. In those terms the
reference is 1 volt, 1 micro-volt (0.000001 volt), and 1 milliwatt
(.001 watt).

The reason you need a reference value is noise prevents you from
measuring 0 Volts and 0 watts accurately so you need to use a small
reference value in its place and so everyone agreed to use these
values.

Definitions a
dBV = 20 * log (volts / 1 )
dBuV = 20 * log ( volts / 0.000001 )
dBm = 10 * log ( power / .001 )

So for example:
30 dBm = 1.0 watt
0 dBm = 0.001 watt (the reference value)
-30 dBm = 0.000001 watt

Most of the time in radio dBm, dBuV, and watts are used.

--
Telamon
Ventura, California

"Telamon" wrote in message
...
Absolute scales would be dBV, dBuV, and dBm. In those terms the
reference is 1 volt, 1 micro-volt (0.000001 volt), and 1 milliwatt
(.001 watt).

The reason you need a reference value is noise prevents you from
measuring 0 Volts and 0 watts accurately so you need to use a small
reference value in its place and so everyone agreed to use these
values.

Given you are discussing use of the dB, I think the above leaves a bit out.
You can't use 0W as your reference because, given the definition of the dB,
you'd need to divide by 0 which, as I'm sure you know, isn't acceptable.

--
73
Brian
www.g8osn.org.uk

"Telamon" wrote in message
...
My point was that some otherwise ambiguous dB scales (at least one) have
a
defined absolute basis.

Like Michael stated dB is a logarithmic reference-less ratio value.

So what is acoustic dB?
http://home.new.rr.com/trumpetb/audio/dBexamp.html
SPL, I should say.

There it lies, x dB on the left scale.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Tim Williams wrote:

"Telamon" wrote in message
...
My point was that some otherwise ambiguous dB scales (at least one) have
a
defined absolute basis.

Like Michael stated dB is a logarithmic reference-less ratio value.

So what is acoustic dB?
http://home.new.rr.com/trumpetb/audio/dBexamp.html
SPL, I should say.

There it lies, x dB on the left scale.


That is a SPL chart with the dB levels referenced to 0 dB SPL =
0.0002 µbar.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

"Michael A. Terrell" wrote:

That is a SPL chart with the dB levels referenced to 0 dB SPL =
0.0002 µbar.

errrr,,, excuse me ...but....

scratches head trying to see

since when has a fraction of microBar had any effect
on an rf link using any sort of modulation ?

\g\g\g\g (zpk) wrote:

"Michael A. Terrell" wrote:

That is a SPL chart with the dB levels referenced to 0 dB SPL =
0.0002 µbar.

errrr,,, excuse me ...but....
scratches head trying to see
since when has a fraction of microBar had any effect
on an rf link using any sort of modulation ?


ahhh..now i see where uBars come in...

ok..

caryy on.

Tim Williams wrote:

"Telamon" wrote in message
...
My point was that some otherwise ambiguous dB scales (at least one) have
a
defined absolute basis.

Like Michael stated dB is a logarithmic reference-less ratio value.

So what is acoustic dB?
http://home.new.rr.com/trumpetb/audio/dBexamp.html
SPL, I should say.

There it lies, x dB on the left scale.


Read this Sound Level Meter manual if you want to understand that
chart.

http://rsk.imageg.net/graphics/uc/rsk/Support/ProductManuals/3304050_PM_EN.pdf


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Tim Williams wrote:

"Michael A. Terrell" wrote in message
...
You need to do a lot of studying on how to use the dB.

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.


I'm not "Patronizing" you. You were using it in the wrong context.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida